1. Field of the Invention
This invention relates to an improved high tensile strength steel wire useful as steel strands of an aluminum conductor, steel reinforced (hereinafter referred to as "ACSR"), a composite construction of aluminum strands surrounding a stranded steel core, which is used for overhead transmission lines.
2. Prior Art of the Invention
Recently the conditions under which the ACSR used for overhead transmission lines must serve have become more severe, and the load, particularly the dynamic load, exerted on the conductors has increased. As a result, it has become essential to improve the mechanical properties, both static strength (e.g. tensile strength) and dynamic strength (e.g. fatigue performance), of the steel wire for use in the ACSR.
The following significant changes in service conditions contribute to the need for better performance characteristics.
(1) Nowadays many long distance overhead transmission lines pass over mountains. Therefore, the span between supports for the lines is longer than before and, also, the difference in height between supports is greater. This means that the weight of the conductor between supports is greater and, also, that the tension exerted on the conductor is greater.
(2) To meet the demand for more power the transmission capacity of the conductor has been increased by enlarging the diameter of the ACSR. This means that the line itself is heavier and this, too, increases the load and tension on the conductor.
(3) In bad weather, especially during the blizzards that often hit mountain areas, a conductor of large diameter accumulates more snow and ice on the surface of the conductor, and this gives more resistance to wind and snow and increases the load and tension on the conductor and, in addition, causes various kinds of vibration resulting in a complicated dynamic tension load on the conductor.
(4) When the conductor is used for ultra-high power transmission (e.g. 750 to 1,000 kV) a rise in temperature (sometimes up to 400.degree.-500.degree. C.) of the ACSR occurs due to the heat generated by electrical resistance (Joule heat).
Under the above-mentioned operating conditions, it is essential that the ACSR have high tensile strength and high resistance to fatigue, not only at normal temperature but also at high temperatures. The tensile strength at high temperatures will hereinafter be referred to as "high temperature strength".
The ACSR presently available are classified into two types. In one type the tension is loaded on both the stranded steel core and the aluminum strands surrounding the core; in the other the tension is held only by the stranded steel core and the aluminum strands are free from the tension. However, with the ACSR of the former type sometimes all the tension has to be held by the steel strands only, not by both the steel strands and the aluminum strands. This is because the length of the aluminum strands increases due to rise in temperature during transmission of electrical power. Therefore, it is not too much to say that the mechanical properties of the ACSR depend entirely on the mechanical properties of the steel strands.
In the prior art ACSR, the galvanized steel wire strand prescribed in JIS G 3537 has been used to prepare the stranded steel core. However, such steel wire does not meet the requirements necessary for enduring the severe service conditions mentioned in Items (1) through (4) above as to tensile strength, fatigue properties and other mechanical properties.
According to the findings of the inventors of this invention, a steel strand which has the following properties can serve satisfactorily as the stranded steel core of the ACSR under the above mentioned severe service conditions:
(a) A tensile strength of 130-260 kgf/mm.sup.2, preferably 180-260 kgf/mm.sup.2 which is higher than that of the conventional one. Such a high tensile strength is required to hold up under the high tension imposed on the conductor.
(b) Fatigue limit .gtoreq.tensile strength (TS).times.0.19.
This fatigue limit is higher than that of the prior art, but such a high fatigue limit is necessary to reduce the risk of the conductor breaking or being damaged under severe service conditions.
(c) TS.sub.(T) .gtoreq.TS.sub.(R) .times.(1.42-0.0028T), preferably TS.sub.(T) .gtoreq.TS.sub.(R) .times.(1.29-0.0019T) wherein, TS.sub.(T) stands for a tensile strength at T.degree. C. in the temperature range of 150.degree. C. to 450.degree. C., and TS.sub.(R) stands for the tensile strength at room temperature.
The above high temperature strength is necessary to ensure that tension on the conductor does not vary significantly at the time of rapid temperature rise of the ACSR.
(d) A proportion of breakage or cracking (hereinafter referred to as "fracture defective ratio") of not more than 50% in a winding test employing a winding radius of curvature of 3.0 times the radius, preferably 1.5 times the radius.
Improvement in winding and unwinding properties in comparison with those of the prior art, as in the above, is desirable so as to prevent the breaking or cracking of the steel wire strands during the production thereof and during the construction of overhead transmission lines.